1. Field of the Invention
This invention relates to an electro-optical device and more particularly to an active-type liquid crystal display device in which clear gradated display levels can be set.
2. Description of the Prior Art
Because of the physical characteristics of the liquid crystal composition, the dielectric constant thereof differs between a direction parallel to the molecule axis and a direction perpendicular to the molecule axis, which is referred to as dielectric anisotropy, therefore the liquid crystals can easily be arranged parallel to or perpendicular to an external electric field. A liquid crystal electro-optical device utilizes this dielectric anisotropy, so that the ON/OFF display is achieved controlling the amount of light transmitted or the amount of light dispersion.
The electro-optical characteristics of a nematic liquid crystal are shown in FIG. 2. The relationship between the applied voltage and the transmissivity (amount of light transmitted) is as follows;
______________________________________ the applied voltage the light transmitted ______________________________________ smaller Va at point A 201 0%; Vb at point B 202 about 30%; Vc at point C 203 about 80%; larger Vd at point D 204 about 100%. ______________________________________
In short, if only points A and D are utilized, the two gradations, black and white, are displayed, and if the rising portion of the electro-optical characteristic curve is utilized, such as at points B and C, an intermediate gradated display is possible.
It was confirmed that Va=2.0 V, Vb=2.18 V, Vc=2.3 V and Vd=2.5 V.
Conventionally, in the case of a liquid crystal electro-optical device with a gradated display utilizing a TFT, the applied gate voltage or voltage applied over the source and drains of the TFT is varied to adjust the voltage, so that an analogue gradated display is obtained.
The gradated display method with a liquid crystal electro-optical device utilizing TFTs is further described below in detail.
An n-channel thin film transistor conventionally utilized in a liquid crystal electro-optical device has the voltage-current characteristic as shown in FIG. 20. In the drawing, numeral 301 designates the characteristic in case of an n-channel thin film transistor using amorphous silicon, while numeral 302 designates the characteristic in case of an n-channel thin film transistor using polycrystalline silicon.
In a conventional gradated display method, by controlling analog voltages to be applied to the gate electrode, drain currents can be controlled and accordingly the resistance value between the source and the drain can be changed. As a result, the strength of the electric fields to be applied to the liquid crystal connected thereto in series can be arbitrarily changed by the division of the resistance, whereby a gradated display is made possible.
Also there is another method, where the gate electrode is connected to scanning signal lines and the voltage between the source and drain is changed, resulting in controlling arbitrarily the electric field value itself to be applied to the liquid crystal.
Both of the above methods are analogue gradated display methods largely relying upon the TFT characteristics. It is however difficult to form numbers of TFTs for matrix composition so as to make all of them have an uniform electric characteristic. Particularly, it is extremely difficult in the present circumstances to finely adjust the intermediate voltage necessary for a gradated display by the present techniques. As can be realized by the electro-optical characteristics of a nematic liquid crystal shown in FIG. 2, a gradated display has to be carried out within 0.32 V, that is, from around 2.08 V, the boundary value of dark condition, to around 2.40 V, the boundary value of light condition. In the case of a gradated display of 16 gradations, the control of the voltage at every 0.02 V in average is required.
On the other hand, when the voltage is controlled at such as point A 201 and point D 204 shown in FIG. 2 where liquid crystal is completely turned ON/OFF, the difference between voltages of 0.5 V or more can be obtained, which will sufficiently ease the variation in TFT characteristics. When, using a plurality of write-in frames, for example 6 frames among 10 frames are turned ON (at 2.5 V) and the remaining 4 frames are turned OFF (2.0 V), the write-in voltage is 2.3 V in average, so that an intermediate gradated display becomes possible.
In this case, however, the drive frequency might be decreased to 30 Hz or lower which is not discernible by the human eyes. Depending on conditions, this becomes a cause of the inferiority of a display such as flicker. Although it is proposed to raise the drive frequency to prevent the above problem, the data transfer speed of a driver IC has its limit up to about 20 MHz.